That is, we take that Btu/ft³·°F = ##{Btu} \over {ft^3\cdot °F}## while according to the standard mathematical rules it actually says Btu/ft³·°F = ##\frac{Btu}{ft^3}\cdot °F##.
The second option avoids this possible confusion by using parentheses.

However when using units it often happens that you neglect this technicality, because you know what the units are supposed to say.

That is, we take that Btu/ft³·°F = ##{Btu} \over {ft^3\cdot °F}## while according to the standard mathematical rules it actually says Btu/ft³·°F = ##\frac{Btu}{ft^3}\cdot °F##.
The second option avoids this possible confusion by using parentheses.

However when using units it often happens that you neglect this technicality, because you know what the units are supposed to say.